This invention relates to procedures for coating aluminum with zinc, and more particularly to so-called zincating procedures for coating aluminum. In a specific sense, it is directed to procedures for coating an aluminum article surface with an adherent layer of zinc that diffuses into the aluminum, when heated, to provide a zinc-enriched layer adjacent the surface for protecting the article against corrosion. The term "aluminum" is used herein to embrace pure metallic aluminum and alloys thereof.
An illustrative application of the present procedures, to which, however, the invention in its broader aspects is not limited, is in the production of brazed aluminum heat exchanger assemblies for use in automobiles or the like. For instance, such assemblies may comprise tubes of the aluminum alloy having the Aluminum Association designation AA 1050 and fins of the aluminum alloy having the Aluminum Association designation AA 3003, joined by brazing, i.e. by heating to a temperature below their melting points in the presence of a brazing alloy of lower melting point and a suitable flux, e.g. as described in U.S. Pat. No. 3,951,328.
Although aluminum alloys have generally satisfactory and even superior properties for use in the aforementioned assemblies, under severe conditions they are sometimes susceptible to pitting corrosion which can shorten the useful lifetime of the affected parts. Thus, in highly corrosive environments, AA 1050 alloy tubes may undergo extensive pitting and consequent early failure.
As described in U.S. Pat. No. 3,268,358, corrosion protection of an aluminum article can be achieved by coating its exposed surface or surfaces with a deposit of metallic zinc and then heating the article (to a temperature below its melting point) so as to cause the zinc to diffuse into a surface-adjacent layer of the aluminum. The resultant zinc-enriched layer, which is anodic to the essentially zinc-free underlying metal, functions sacrificially to protect the article against corrosion. In the case of a brazed assembly as referred to above, the heat of the brazing operation itself can serve to effect the requisite diffusion, provided that the component or components to be protected are adequately coated with zinc prior to brazing; but the attainment of a satisfactory zinc coating has heretofore presented difficulties.
It is known (as described at pp. 630-31 of vol. 2 of the Metals Handbook, 8th Edition, American Society for Metals, 1964) to apply zinc to an aluminum surface by a so-called zincating treatment, wherein (ordinarily after activation of the surface as with a dilute aqueous NaOH solution and/or with an acid bath treatment) the surface is exposed to an alkaline zincate solution. A conventional zincating procedure employs an aqueous solution containing 100 grams of zinc oxide per liter and 500 grams of sodium hydroxide per liter, at a temperature of 20.degree. C. or less. The hydroxide (as at present believed) removes aluminum oxide initially present on the aluminum surface, and deposit of zinc on the surface proceeds by chemical displacement, aluminum ions replacing zinc ions in the solution. At the outset, in such conventional procedures, zinc deposition may be relatively rapid, but usually within a period of not more than about a minute, when the amount of zinc deposited is typically not more than about one gram per square meter, the rate of deposition becomes extremely slow and the process is ordinarily terminated.
Zincating treatments of the described conventional type are used to prepare aluminum surfaces for subsequent electroplating, and are satisfactory for this purpose. The amount of zinc deposited by zincating procedures as heretofore known, however, is inadequate in at least many instances to provide the requisite supply of zinc for subsequent diffusion to protect an aluminum article effectively against pitting corrosion; that is to say, the layer of zinc thus applied is insufficient to diffuse to the required depth (when the coating article is heated) and to provide the required zinc concentration at the aluminum surface after diffusion, for satisfactory corrosion protection. A typical zinc deposit produced by conventional zincating may diffuse only to a depth of about 60 microns and provide a zinc concentration at the surface as low as 0.2-0.5% by weight. Furthermore, even if it is attempted to achieve a heavier coating of zinc with conventional zincating procedures as by extending the treatment period, the time required to develop a desired amount of deposition (e.g. 10 grams per square meter or more) is uneconomically protracted, being typically as much as 30 minutes or even longer; the results obtained with given conditions are not reliably reproducible; and the produced coatings tend to be very uneven and/or so poorly adherent to the aluminum surface that substantial amounts of the coating material may become lost during handling prior to heating for diffusion. Consequently, notwithstanding the simplicity and convenience of zincating procedures, they have had serious drawbacks for such purposes as depositing zinc for subsequent diffusion for corrosion protection.